Fixed #19508 -- Used uri_to_iri.

This is a PR for ticket #19508 for which another PR #2919 was opened by me a few days ago.
This PR implements `uri_to_iri` as per RFC and changes the `PATH_INFO` at the very time when `environ` is being set contrary to the previous one which corrected the received url from `environ`.
Even in werkzeug's `iri_to_uri`, there are sevaral parameters but not in ours. Here also I didn't felt the need, so I didn't. Though I will look once again and see if any need is there?